For semiconductor devices, the technique for controlling the interface between an insulating film and a semiconductor is critically important. In a case where the insulating film is a tunnel insulating film used in a flash memory, the insulating film serves not only to control the conduction of carriers flowing in the semiconductor but also to control exchanges of electrons between a charge storage film and the semiconductor layer.
As the method of forming the tunnel insulating film on the semiconductor layer, a process to form an insulating film through deposition at 800 to 900° C. or lower by CVD (Chemical Vapor Deposition), HTO (High Temperature Oxide), ALD (Atomic Layer Deposition), or the like is becoming commoner than a so-called thermal oxidation process to expose the surface of the semiconductor layer to an oxygen molecular gas at atmospheric pressure and at a high temperature of 900° C. or higher. This is because the structure of each flash memory is changing from a conventional flat-type structure to a three-dimensional structure, and the distances and volumes between devices are becoming shorter and smaller as miniaturization progresses. In such a trend, changes in volume at the time of high-temperature treatment cannot be absorbed by devices, and the manufacturing process needs to be a low-temperature process with low stress.
A three-dimensional structure also differs from a flat-type structure in that the semiconductor layer is deposited after the tunnel insulating film is deposited. For example, JP-A 2007-266143(KOKAI) discloses a technique by which a NAND string is perpendicular to an in-plane direction, not parallel to an in-plane direction as in conventional cases so that the control gate electrode is formed first, and the channel Si layer is formed lastly. However, since the formation temperatures of a depositional insulating film and a semiconductor layer, or of a depositional insulating film and a depositional semiconductor layer, are low, the interface state density is high. Therefore, the mobility and the reliability are lower than those of conventional structures subjected to high-temperature treatments, and the characteristics easily vary. This leads to degradation of write and erase characteristics, and to difficulties in performing multi-valued processing due to increases in threshold value variations.
The depositional semiconductor layer and the depositional insulating film are used in Thin Film Transistors (TFTs) for liquid crystal displays, and the channel layer is formed by depositing amorphous silicon or polysilicon on an insulating film such as a glass substrate. In that case, however, the mobility is low as in the above described cases. Therefore, instabilities stem from the facts that it is difficult to form high-speed circuits, and the threshold voltage varies with the elapsed time, the gate voltage, and the temperature. In that case, it is also necessary to increase the mobility and reliability.
A large number of defects such as Si dangling bonds exist at the interface between a Si layer and an insulating film (such as a SiO2 film) formed on the Si layer. Those defects form interface states. In a MOSFET (Metal-Oxide-Silicon Field Effect Transistor) that is a typical structure of, for example, a switching device, those defects result in decreases in mobility and reliability, and the switching characteristics are degraded. To solve this problem by a conventional technique, a MOS structure is subjected to a heat treatment in a hydrogen atmosphere, and the Si dangling bonds are inactivated through hydrogen termination or formation of Si—H bonds.
In recent years, however, it has become clear that the hydrogen termination has a problem. Specifically, the Si—H bonds form a large dipole, and cause carrier scattering. In a flash memory, hydrogen is detached from the Si—H bonds under stress of an electric field, and the program voltage Vgm is varied. Therefore, in a stack structure including a SiO2 film and a Si layer having a large number of interface states, a termination technique that replaces the hydrogen termination is required. Particularly, as for the tunneling films that are components of nonvolatile memories, the oxidation temperature for CVD (Chemical Vapor Deposition) or the like is becoming lower to cope with three-dimensional structures, and the interface state density prior to hydrogen termination is ever increasing. Therefore, higher voltages are generally used to lower the mobility and increase the on-state current Ion. The higher voltages reduce the reliability of each device and hinder multi-valued operations.